Heretofore, to obtain radiation images, a so-called radiographic method has been employed. However, a method has been developed in which radiation images are visualized without using silver salts. Namely, the following method is disclosed. Radiation which has passed through an object is absorbed by a phosphor. Subsequently, the phosphor is excited utilizing a certain type of energy, and radiation energy which is stored in the phosphor is allowed to be emitted as fluorescence which is detected to form images.
Known as a specific method is a radiation image converting method in which a panel comprising a support having thereon a stimulable phosphor layer is employed and either visible light or infrared radiation, or both, are employed (refer to U.S. Pat. No. 3,859,527).
Developed as radiation image converting methods employing high luminance and high speed stimulable phosphors are, for example, a radiation image converting method using BaFX:Eu2+ (wherein X is Cl, Br, or I) based phosphors described in Japanese Patent Application Open to Public Inspection No. 59-75200 and a radiation image converting method using alkali halide phosphors described in Japanese Patent Application Open to Public Inspection No. 61-72087. Further, developed are alkali halide phosphors containing metals such as Tl+, Ce3+, Sm3+, Eu3+, Y3+, Ag+, Mg2+, Pb2+, and In3+ as described in Japanese Patent Application Open to Public Inspection Nos. 61-73786 and 61-73787.
Further, in recent years, in analysis of diagnostic images, a radiation image converting panel with higher sharpness is still being sought. As a means to enhance sharpness, a trial has been made in which for example, the shape of a stimulable phosphor itself is controlled to enhance speed as well as sharpness.
One method in such trails includes, for example, a method in which a stimulable phosphor layer comprised of minute pseudo-columnar blocks, described in Japanese Patent Application Open to Public Inspection No. 61-142497, is employed which is formed by accumulating a stimulable phosphor onto a support having a fine uneven pattern.
Further, proposed methods include a method to use a radiation image converting panel having a stimulable phosphor layer, in which, as described in Japanese Patent Application Open to Public Inspection No. 61-142500, cracks between columnar blocks, which are prepared by accumulating a stimulable phosphor on a support having a fine pattern, are subjected to a shock treatment so that the aforesaid cracks are allowed to grow and further, a method to use a radiation image converting panel in which, as described in Japanese Patent Application Open to Public Inspection No. 62-39737, a stimulable phosphor layer formed on a support is subjected to formation of cracks on the surface side to be pseudo-columnar, and further, a method in which, as described in Japanese Patent Application Open to Public Inspection No. 62-110200, a stimulable phosphor layer having voids is formed on a support, employing vacuum evaporation, and subsequently voids are allowed to grow by a thermal treatment so that cracks are provided.
Still further, Japanese Patent Application Open to Public Inspection No. 2-58000 describes a radiation image converting panel having a stimulable phosphor layer in which thin and long columnar crystals, having a definite slope with respect to the normal direction of a support, are formed on the aforesaid support, using a vapor phase growth method (being a vapor phase sedimentation method).
These methods which control the shape of the stimulable phosphor layer are characterized in that in all cases, by allowing the stimulable phosphor layer to be columnar, it is possible to minimize diffusion of stimulating light or stimulated luminescence in the lateral direction (namely, light reaches the support surface after being repeatedly reflected on the interface of cracks, i.e., columnar crystals), whereby it is possible to markedly enhance image sharpness.
Recently, a radiation image converting panel has been proposed in which alkali halide, such as CsBr, is incorporated as a host and Eu is used as an activator.
Particularly, by employing Eu as an activator, it has become possible to achieve enhancement of X-ray conversion efficiency, which has been considered to be impossible.
However, Eu exhibits markedly high thermal diffusion as well as high vapor pressure under vacuum, whereby problems occurred in which Eu was unevenly distributed in the host due to ease of scattering in the host. As a result, it has been difficult to achieve high X-ray conversion efficiency through activation employing activators, and thus commercial viability has not been realized.
Further, in techniques in which activation is carried out employing rare earth atoms such as Eu, it has been difficult to uniformly distribute the desired activator only by controlling its vapor pressure characteristics during formation of a vacuum evaporated layer.
Specifically, when a stimulable phosphor layer is prepared employing the aforesaid vapor phase method (sedimentation), application of several thermal processes such as heating of raw materials, heating of the support during vacuum evaporation, and annealing after the layer formation (relaxation of distortion of the substrate) cause non-uniform distribution of activators.
However, all these heating processes have been essentially required to provide durability to the stimulable phosphor layer.
Accordingly, improvements in luminance, sharpness, as well as uniform distribution of activators of the radiation image converting panel have been sought on the market.
Moreover, in crystals in which alkali halide is used as a main component, when a composition is varied to increase X-ray absorption, distortion of phosphor crystals increases. As a result, even though high luminance is exhibited due to the presence of many emission levels, emission distribution in terms of emission wavelengths is broadened, resulting in broad emission. The resulting broad emission is markedly affected when Eu is employed as the activator. As a result, even though resulting in high luminance, delayed luminescence-response is degraded due to distribution broadening at emission levels.
When alkali halide is used in a CR detector as a radiation image capturing system, problems of the aforesaid delayed luminescence characteristics are pronounced. Specifically, in the radiation image capturing system, problems occur in which it is necessary to carry out reading a definite time till reading after X-ray exposure, and with regard to effects of photostimulated delayed luminescence, contrast during reading is degraded depending on the delayed luminescence. In addition, it was found that the reading rate (being a reading cycle or use frequency) was adversely affected.
Accordingly, enhancement in luminance as well as sharpness of the radiation image conversion panel and improvement of delayed luminescence characteristics (instantaneously emitted delayed-luminescence as well as photostimulated delayed luminescence), which matches an increase in processing rate, are continuously demanded on the market.